1. Field of the Invention
The present invention relates to a light sensing unit and a related light sensing circuit, and more particularly, to a light sensing unit and a related light sensing circuit capable of realizing high signal-to-noise ratio, high light sensitivity and high dynamic range in an image sensor.
2. Description of the Prior Art
An image sensor, as a device capable of converting an optical image into an electronic signal, is widely applied in various electronic products such as digital cameras, medical image products, monitoring equipment and video telephones. Currently available image sensors are divided into charge coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors. With advances in integrated circuit process technology, the CMOS image sensor has become more popular than the CCD image sensor in the image sensor market.
The CMOS image sensors are further divided into passive pixel sensors (PPS) and active pixel sensors (APS) according to the circuit structure of pixels. The difference is that in the APS, the signals transmitted to back-end circuits pass through an active element in advance. The APS may have different structures due to transistor counts, such as 3T structure (with 3 transistors) or 4T structure (with 4 transistors). In a CMOS image sensor, image sensing is performed in the following manners: a light sensing element in each pixel performs light sensing; the light sensing element then generates carriers corresponding to sensed light intensity and transmits the carriers to a floating node; and the back-end circuit reads out the number of carriers in the floating node to obtain a light sensing result.
“Dynamic range” is the maximal bottleneck in development of modern CMOS image sensors. People eye can always be adaptive to a dynamic range up to more than 200 dB, but most available CMOS image sensors cannot achieve this requirement; hence, most of image sensor manufacturers are striving to enhance the dynamic range of image sensors. A common method of enhancing dynamic range is to perform multiple exposures, but this method cannot achieve high signal-to-noise ratio (SNR) and high light sensitivity simultaneously since the carrier transmission efficiency in the light sensing elements and the readable signal range of the output circuit are limited. Another method of enhancing dynamic range is to enlarge the floating node capacitors. Although this method may achieve high SNR, the increasing of dynamic range is accompanied by a tradeoff of decreasing light sensing area; that is, a large amount of light sensing area is sacrificed due to the requirement of high dynamic range, such that high light sensitivity cannot be achieved at the same time. Thus, there is a need to provide an image sensor capable of high dynamic range, high light sensitivity and high SNR simultaneously.